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A Numerical Modeling Investigation of a Case of Polar Airstream Cyclogenesis over the Gulf of Alaska

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  • 1 Department of Atmospheric Sciences, University of California, Los Angeles, Los Angeles, California
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Abstract

The primary objectives of this study were 1) to test the ability of a high-resolution (40 km) limited-area model to successfully simulate a meso-α-scale case of polar airstream cyclogenesis, 2) to examine the effects of various physical conditions and dynamical processes on the storm development, and 3) to examine the evolving structure of the system. Principal findings were as follows.

  1. The control experiment, which utilized a 40-km horizontal grid increment, a Kuo-type cumulus parameterization, and surface sensible and latent heat fluxes, produced a small-scale cyclone with a central pressure of 996 mb at hour 24 (as compared with a subjectively analyzed pressure of 994 mb) with the position of the low center within 100 km of the analyzed location.

  2. Cyclogenesis, albeit significantly weaker, did occur in an adiabatic simulation. This appeared to result from the adiabatic forcing associated with a migrating upper-level short-wave trough.

  3. Strong ascent in the control simulation occurred near the surface low center and in a narrow plume on the warm-air side of a baroclinic zone that developed over the Bering Sea downwind of the polar ice sheet. This zone appeared structurally similar to that of the intense warm-frontal zone seen in explosive cyclones (near the low center). Removal of the effects of latent heat release significantly diminished the intensity of this rising motion and resulted in an expansion of its horizontal scale. A simulated increase in the sea surface temperature resulted in stronger rising motion and in a smaller and more intense cyclone.

  4. Surface fluxes and the attendant latent heat release in the plume of rising motion played a significant role in the intensification of the cyclone.

Abstract

The primary objectives of this study were 1) to test the ability of a high-resolution (40 km) limited-area model to successfully simulate a meso-α-scale case of polar airstream cyclogenesis, 2) to examine the effects of various physical conditions and dynamical processes on the storm development, and 3) to examine the evolving structure of the system. Principal findings were as follows.

  1. The control experiment, which utilized a 40-km horizontal grid increment, a Kuo-type cumulus parameterization, and surface sensible and latent heat fluxes, produced a small-scale cyclone with a central pressure of 996 mb at hour 24 (as compared with a subjectively analyzed pressure of 994 mb) with the position of the low center within 100 km of the analyzed location.

  2. Cyclogenesis, albeit significantly weaker, did occur in an adiabatic simulation. This appeared to result from the adiabatic forcing associated with a migrating upper-level short-wave trough.

  3. Strong ascent in the control simulation occurred near the surface low center and in a narrow plume on the warm-air side of a baroclinic zone that developed over the Bering Sea downwind of the polar ice sheet. This zone appeared structurally similar to that of the intense warm-frontal zone seen in explosive cyclones (near the low center). Removal of the effects of latent heat release significantly diminished the intensity of this rising motion and resulted in an expansion of its horizontal scale. A simulated increase in the sea surface temperature resulted in stronger rising motion and in a smaller and more intense cyclone.

  4. Surface fluxes and the attendant latent heat release in the plume of rising motion played a significant role in the intensification of the cyclone.

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